1. Field of the Invention
The present invention relates to a fluid lubricated bearing device for allowing a lubricating oil film to be produced in a radial bearing gap to provide non-contact support of a rotating member. For example, this bearing device is suitable for use with: spindle motors incorporated into information apparatus, for example, a magnetic disk device such as an HDD or FDD, an optical disk device such as a CD-ROM, CD-R/RW, or DVD-ROM, a magneto-optical disk device such as an MD or MO; scanner motors incorporated into a copier, laser beam printer (LBP), or barcode reader; and small motors incorporated into electric appliances such as an axial fan.
2. Description of the Related Art
Each of the motors of the aforementioned types is required to satisfy higher speed, lower cost, and lower acoustic noise requirements in addition to higher rotational accuracy. One of the components that are decisively responsible for these requirements is a bearing for supporting the spindle of these motors. In recent years, use of those fluid lubricated bearings have been attempted or actually made which have sufficiently good properties to satisfy the aforementioned requirements. The fluid lubricated bearings of these types are largely divided into two categories: one with means for generating dynamic pressure to provide dynamic pressure to the lubricating oil in bearing gaps or the so-called hydrodynamic bearing, and the other without means for generating dynamic pressure or the so-called fluid cylindrical bearing (with its bearing surface shaped in a perfect circle).
FIG. 7 is an example configuration of a spindle motor of an information apparatus incorporating a hydrodynamic bearing device 31. This spindle motor, used for a disk drive device of a DVD-ROM or the like, is provided with a fluid lubricated bearing device 31 for rotatably supporting an axial member 32, a support member 34 attached to the axial member 32 to support an object to be driven such as an optical disk 33 (a turntable in this illustrated example), and a motor stator 35 and motor rotor 36 which are opposed to each other via a radial gap present therebetween.
The fluid lubricated bearing device 31 mainly consists of a housing 21 having an opening portion at one end and a bottom portion at the other end, a cylindrical bearing member 22 secured to the inner peripheral surface of the bearing member 22, the axial member 32 inserted into the inner peripheral surface of the bearing member 22, a thrust plate 23 providing on the bottom portion of the housing 21, and a seal member 24 attached to the opening portion of the housing 21. On the inner peripheral surface of the bearing member 22 or the outer peripheral surface of the axial member 32, there are provided grooves for generating dynamic pressure (dynamic pressure generating grooves). On the other hand, a lubricating oil is loaded into the space inside the housing 21.
The stator 35 is attached to the outer periphery of the housing 21 of the fluid lubricated bearing device 31, while the rotor 36 is attached to the support member 34. Flowing an electric current through the stator 35 causes the rotor 36 to rotate due to exciting force established between the stator 35 and the rotor 36, thereby allowing the support member 34 and the axial member 32 to rotate integrally.
The rotation of the axial member 32 causes the dynamic pressure generating grooves to produce a dynamic pressure action of the lubricating oil in the radial bearing gap between the inner peripheral surface of the bearing member 22 and the outer peripheral surface of the axial member 32, thereby providing radial non-contact support to the outer peripheral surface of the axial member 32. In addition, the end surface of the other end of the axial member 32 (the lower side in FIG. 7) is supported in the direction of thrust by means of the thrust plate 23.
In addition to the one mentioned above, such a bearing device is also available for providing non-contact support to the axial member in the radial and thrust direction by means of the dynamic pressure action produced in both the radial and thrust bearing gaps. In general, in the bearing of this type, a space defined by a thrust carrying face formed on the bottom portion of the housing and an end face of the axial member opposed thereto is formed in a sealed arrangement. Accordingly, on the outer periphery of the axial member, there are formed axial grooves (circulation grooves) provided with openings at both the end faces thereof to expose this seal space to ambient air.
The lubricating oil is loaded into the space inside of the housing 21 generally at the time of assembly of a spindle motor before the axial member 32 is installed, and the axial member 32 is built therein after the lubricating oil has been loaded. For this reason, it cannot be avoided to ingest ambient air into the inner space of the housing 21. Additionally, the air inside the inner space of the housing may be thermally expanded or contracted due to variations in ambient temperature, heat generated by the motor, or variations in atmospheric pressure during operations at high altitudes or transportation by airfreight. This may cause the lubricating oil to be pushed out of the seal space defined by the inner peripheral surface of the seal member 24 and the outer peripheral surface of the axial member 32 to migrate outside thereof. In particular, when the motor is operated in its inverted position (i.e., the opening portion of the housing 21 is oriented downwardly) or in its horizontal position (i.e., the opening portion of the housing 21 is oriented horizontally), the lubricating oil may readily flow toward the opening-portion and stay there, thereby increasing the likelihood of leakage of the lubricating oil.
For the reasons mentioned above, motors incorporating the conventional fluid lubricated bearing device 31 were unreliable during operations in their inverted and horizontally oriented positions, and thus limitation was imposed on their operable positions.
Furthermore, in the fluid lubricated bearing device 31 configured as described above, the thrust bearing portion employs the end face at the other end of the axial member 32 by means of the thrust plate 23, and the axial member 32 is pushed against the thrust plate 23 by means of the magnetic force established between the stator 35 and the rotor 36, thereby restricting the axial movement of the axial member 32 towards the one end (the upper side in FIG. 7). However, when a shock load exceeding the aforementioned magnetic force is imposed on the motor or the motor is operated in an inverted or horizontally oriented position, the axial member 32 may move axially towards the one end relative to the housing 21 to dislodge from the housing 21.
In the manufacturing process of the bearing member, a sleeve-shaped sintered metal is sized in a die to form the bearing member in a predetermined size. After the sizing followed by the removal from the die, the outer periphery of the bearing member expands radically outwardly due to spring-back. However, the circulation groove portion is not in contact with the die during the sizing and therefore has not been pressurized radically inwardly, thereby being provided with a less amount of spring-back when compared with other portions after the bearing member has been removed from the die. Accordingly, as shown in FIG. 8, after the sizing, the outer and inner periphery of the bearing member 22 is not of a complete round but of an uneven cross section with the vicinity of the circulation grooves 25 being reduced in diameter. Conventionally, the circulation grooves 25 are frequently formed at two positions on the outer periphery of the bearing member 22 (at opposite positions different by 180 degrees). In this case, the resulting cross section obtained after the sizing displays an ellipse with its minor axis being in the direction connecting between the circulation grooves 25.
However, such an elliptical shape, if remains unchanged, causes a narrow portion (in the direction of the minor axis) and a wide portion (in the direction of the major axis) to be formed in the radial bearing gap between the inner periphery of the bearing member 22 and the outer periphery of the axial member 32. In this case, the lifting effect provided by the hydrodynamic pressure on the shaft is reduced in the wide portion of the radial bearing gap, thereby causing axial runout due to a degradation in bearing rigidity in the direction of the elliptical major axis and thus probably having an adverse effect on NRRO or the like.
An object of the present invention is to provide a fluid lubricated bearing device and a motor that incorporates the same, which can be operated or transported with stability at any attitude without lubricating oil leakage out of the housing due to expansion and contraction of air remaining in the inner space thereof, under high and low temperature environments, or reduced pressure environments during operations at high altitudes or transportation by airfreight.
Another object of the present invention is to restrict the axial movement of the axial member relative to the housing towards one end thereof, thereby preventing the axial member from dislodging from the housing.
Another object of the present invention is to eliminate differences in bearing rigidity in all directions that is caused by deformations of the bearing member after having undergone spring-back, thereby ensuring high rotational accuracy.